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Most of the time, composting is as simple as taking out the trash. In our house, we collect our kitchen scraps over a day or two and then toss them right onto the compost pile. Yard waste, such as spent annuals, grass clippings from the lawn and weeds from the garden, all find their next incarnation in our ongoing, living compost pile.

Composting is like this. Even though there’s solid science behind the process, most of the time, success is achieved by just tossing in what needs to be composted. Occasionally, though, this “toss it and forget it” method of composting doesn’t work and can lead to a rather unpleasant compost smell. A bad smell coming from the compost pile is a tell-tale sign that something is wrong. Let’s explore a few reasons why that might be.

1. The Compost Is Compacted Or Lacking Oxygen

Without a properly aerated compost pile, the bacteria and microorganisms that break down the raw material within it can’t do their jobs. These good guys, called aerobic bacteria—bacteria that need oxygen—help to break down the raw material in a compost pile. In the absence of good airflow and a healthy aerobic bacteria population, anaerobic bacteria take a foothold, and they produce some pretty foul odors through their metabolic processes. Hence, you get compost smell. To some degree, all of these microbes call the compost pile home. As with anything, keeping the balance in check is important to a successful compost pile.

Take a look at your pile’s location. Is it in a very wet, damp or shady area? This could lead to improper airflow. Other times, too much dense material (think matted wet leaves), prevents oxygen from finding it’s way into the heart of the compost. This may be your issue if your compost smells sulfurous, like putrid or rotting eggs.

The solution to a soggy compost is simple: Turn the pile. Tumbling composters are usually on an axel that makes rolling or turning simple, with minimal strength required. Plein air composts will require a pitchfork or rake to properly expose the depths of the pile to some good, fresh air. Turning a smelly compost pile doesn’t need to happen daily. Once a week should suffice until the compost smell subsides, and then turn it over as needed.

2. The Compost Is Too Wet

Most compost piles are located outdoors, and the most successful piles are going to break down properly with exposure to the elements: wind, rain and sun. If one of these elements is off balance (say, from the compost being placed in a shady location where it doesn’t get much heat from the sun), the contents of a pile can sit stagnant, growing smelly and soggy and feeding those anaerobic bacteria, rather than breaking down and decaying properly. A too-wet compost pile often resembles (in appearance and odor) one that isn’t getting enough oxygen.

In addition to turning or “fluffing” the compost, make sure your pile is located in a spot that receives at least as much sun as your garden—6 to 8 hours per day is ideal. If the pile doesn’t get enough air circulation, this can result in compost smell. So include turning the pile as one of your new weekly chores and consider moving the pile’s location if it doesn’t get enough sun.

3. The Greens & Browns Are Off Balance

A general rule of thumb when composting is to keep your “greens” (nitrogen-rich materials) and “browns” (carbon-rich materials) in perfect harmony. That looks like a green-brown ratio of anywhere from 50:50 to 20:80. The difference between the ratios (and everything in between) depends largely on your microclimate and how you manage your compost pile.

Piles too heavy in green materials (food scraps, grass clippings, coffee grounds, etc.) will leave your pile smelling of ammonia. If your pile smells this way, increase your browns (shredded newspaper, pine needles, straw, etc.) and aerate by turning your pile.

Sometimes, the problem isn’t just the ratio, but the placement of the nitrogen and carbon materials. Too many greens together and too many browns together will keep the pile from breaking down properly. Layer the materials lasagna-style, and spread them out evenly, keeping a nose to the smell.

Finding the right balance of greens and browns may take a little trial and error. Increase your carbon a little at a time, and turn as needed, to find the sweet spot.

4. You Threw In Meats, Dairy, Heavy Oils Or Animal Fat

These items truly have no business being in a compost pile, at least not in a home pile used for the garden. In small, backyard composts, especially if you live in urban or suburban areas, keep meats, dairy, greasy take-out leftovers, and other animal and fatty products out of the compost pile. Keeping non-plant matter out of the bin is one way to guard against compost smell, and it keeps neighborhood cats, dogs, rodents and raccoons out of the bin, too.

5. There’s A Lack Of Microorganisms

On very rare occasion, a pile will completely lack bacteria in sufficient populations to break down the new materials. This may be the case if the compost pile is isolated from natural sources of microbes, such is the case with tumbling composters, where the material is often sealed in a plastic tub up off the ground, or piles placed on tarps, where the material doesn’t touch the earth. A pile lacking microorganisms will not only smell, it won’t heat up and it won’t break down.

Ditch the tumblers and plastic sheets, and place the compost bin on solid ground to get some good bugs into the pile. You may also choose to introduce microorganisms in the form of fresh compost or organic soil. This method is similar to using bacterial culture to start a new batch of yogurt.

Remember, your compost is a living, breathing, thriving ecosystem. Healthy compost shouldn’t smell. When it’s breaking down properly, it actually smells rather nice—like dirt and grass after a rainstorm. Let that be your goal, and you’ll have beautiful “black gold” to add to the garden in no time.


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